US Patent 7708396 Photochromic phase change inks

An ink set is made up of at least three differently colored phase change inks, for example cyan, magenta and yellow, wherein at least one of the colored phase change inks is a photochromic ink that contains a photochromic material. Upon exposure to activating radiation such as ultraviolet light, the at least one photochromic ink changes to a color different from the color of the photochromic ink prior to exposure to the activating radiation.